Because of a rapid increase in video content services typified by video streaming services in addition to a rapid spread of advanced terminals and the like, the transmission capacity in networks is drastically increasing.
Against such a background, it is studied nowadays to introduce a multi-layer network composed of a plurality of layers to communication carrier networks. An example of the multi-layer network is a configuration in which a packet network in an upper layer is combined with an optical network in a lower layer. Here, among adjacent layers hierarchized as service networks, a network located at a relatively upper level is referred to as an upper layer, and a network located at a relatively lower level is referred to as a lower layer.
The upper-layer network is a network configured by using the Internet protocol (IP) or the multi-protocol label switching (MPLS) technologies, for example. The IP network is characterized by efficient use of network resources due to the statistical multiplexing effect. In contrast, the optical network of the lower-layer network is suitable for long-haul high-capacity transmission. In general, the network is controlled independently with respect to each layer. However, it is expected to maximize efficiency in the use of the network resources and reduce operational costs by integrating these two types of network layers and controlling the network efficiently in response to the traffic demand.
In order to meet a growing traffic demand, it is being studied, in addition to the introduction of the multi-layer network mentioned above, to introduce new optical network concepts and network operational methods. Such examples include elastic network technologies and dynamic network operational technologies.
In the optical network of the lower-layer, the elastic network technology is introduced increasingly by which the network can be utilized more flexibly. The elastic network technology is a technology that enables the transmission with minimum frequency band for the transmission distance and the transmission throughput by making variable the modulation scheme in the optical layer, which was fixed conventionally. This makes it possible to maximize the usage efficiency of optical network resources such as wavelength resources in an optical fiber. The greatest characteristic of the elastic network technology is that the transmission granularity in the optical layer can be improved by introducing the concept of frequency slot with fine granularity of 12.5 GHz instead of the conventional fixed grid such as 100 GHz and 50 GHz. Hence, it is thought that a multicarrier transmission scheme become mainstream in the future which transmits signals through a plurality of physical media such as a plurality of optical carriers in the optical layer.
With regard to the network operational technology, it is expected to operate the network dynamically in contrast to the conventional fixed network operation. This is accounted for by the increase in a variation of the traffic of a client to be accommodated in networks. It is expected that the dynamic operation of optical networks can improve the network usage efficiency.
However, it is pointed out that the introduction of the above-mentioned elastic network technology and dynamic network operational technology causes a fragment of wavelength bands to arise. This produces the problem that a path with a long-haul route cannot be secured in the same wavelength due to the occurrence of wavelength fragmentation even though the introduction of the elastic network technology enables the accommodation efficiency of the entire network to improve. The fragmentation means a state in which unused wavelength regions are fragmentated in the wavelength usage situation of each link constituting an optical network. Technologies to resolve the wavelength fragmentation include a wavelength defragmentation technology. In general, the wavelength defragmentation technology is a technology to improve the efficiency in the wavelength usage by relocating one wavelength occupying a particular link in an optical network to the other wavelength.
Patent Literature 1 discloses an example of such wavelength defragmentation technologies. A related frequency assignment apparatus described in Patent Literature 1 selects a frequency and a route connecting a start point and an end point of an optical signal and includes a route/frequency calculation result storage means, a common free frequency information generation means, a free frequency state evaluation means, and a frequency and route determination means.
The route/frequency calculation result storage means stores route and frequency calculation results. The common free frequency information generation means extracts fibers connected to each other, and performs logical operation for logical information representing free frequency states of each of the extracted fibers so as to generate logical information on free frequency states common to fibers. The free frequency state evaluation means provides an evaluation value for the free frequency states based on the generated free frequency information common to fibers, in consideration of consecutiveness of free frequencies in the free frequency state common to fibers. The frequency and route determination means determines a frequency and passing fibers to be set as a communication route using the evaluation value calculated in the free frequency state evaluation means as a criterion, and stores the frequency and the passing fibers in the route/frequency calculation result storage means.
It is said that the configuration, according to the frequency assignment apparatus described in Patent Literature 1, makes it possible to effectively suppress occurrence of fragmentation in a transparent type optical path network, and to optimize utilization efficiency of wavelength (frequency) resources.
Related technologies are described in Patent Literature 2 to Patent Literature 4.